The majority of present day integrated circuits (ICs) are implemented by using a plurality of interconnected field effect transistors (FETs), also called metal oxide semiconductor field effect transistors (MOSFETs), or simply MOS transistors. A MOS transistor includes a gate electrode as a control electrode and spaced apart source and drain regions, defining a transistor channel, between which a current may flow. A control voltage applied to the gate electrode controls the flow of current through a channel between the source and drain electrodes. Complementary MOS (CMOS) devices include a plurality of N-channel MOS (NMOS) transistors and a plurality of P-channel (PMOS) transistors. During the fabrication of complex integrated circuits using, for instance, MOS technology, millions of transistors, e.g., NMOS transistors and/or PMOS transistors, are formed on a substrate including a crystalline semiconductor layer.
In a field effect transistor, the conductivity of the channel region, i.e., the drive current capability of the conductive channel, is controlled by the gate electrode formed adjacent to the channel region and separated therefrom by a thin gate insulation layer. The conductivity of the channel region, upon formation of a conductive channel due to the application of an appropriate control voltage to the gate electrode, depends upon, among other things, the dopant concentration, the mobility of charge carriers therein and, for a given extension of the channel region in the transistor width direction, the distance between the source and drain regions, which is also referred to as the channel length of the transistor. Hence, in combination with the capability of rapidly creating a conductive channel below the insulating layer upon application of the control voltage to the gate electrode, the conductivity of the channel region substantially affects the performance of MOS transistors. Thus, since the speed of creating the channel, which depends in part on the conductivity of the gate electrode, and the channel resistivity substantially determine the characteristics of the transistor, the scaling of the channel length, and associated therewith the reduction of channel resistivity and the increase of gate resistivity, are dominant design efforts used to increase the operating speed of integrated circuits.
For many early device technology generations, the gate electrode structures of most transistor elements have included a plurality of silicon-based materials, such as a silicon dioxide and/or silicon oxynitride gate insulation layer, in combination with a polysilicon gate electrode. However, as the channel length of aggressively-scaled transistor elements has become increasingly smaller, many newer generation devices employ gate electrode stacks including alternative materials in an effort to avoid the short-channel effects that may be associated with the use of traditional silicon-based materials in reduced channel length transistors. For example, in some aggressively-scaled transistor elements, which may have channel lengths of the order of approximately 14 nm to about 32 nm, gate electrode stacks including a so-called high-k dielectric/metal gate (HK/MG) configuration have been shown to provide significantly enhanced operational characteristics over the heretofore more commonly used silicon dioxide/polysilicon (SiO/poly) configurations.
Depending on the specific overall device requirements, several different high-k materials (i.e., materials having a dielectric constant, or k-value, of approximately 3.7 or greater) have been used with varying degrees of success for the gate insulation layer in a HK/MG gate electrode structure. For example, in some transistor element designs, a high-k gate insulation layer may include tantalum oxide (Ta2O5), hafnium oxide (HfO2), zirconium oxide (ZrO2), titanium oxide (TiO2), aluminum oxide (Al2O3), hafnium silicates (HfSiOx), and the like. Furthermore, one or more non-polysilicon metal gate electrode materials (i.e., a metal gate stack) may be used in HK/MG configurations so as to control the work function of the transistor. These metal gate electrode materials may include, for example, one or more layers of titanium (Ti), titanium nitride (TiN), titanium-aluminum (TiAl), aluminum (Al), aluminum nitride (AlN), tantalum (Ta), tantalum nitride (TaN), tantalum carbide (TaC), tantalum carbonitride (TaCN), tantalum silicon nitride (TaSiN), tantalum silicide (TaSi), tungsten (W), and the like. Additionally, a work function modifying material, such as lanthanum (La) or Al, may be disposed in between the gate insulation layer and the metal gate electrode.
Of the above-noted gate insulation layer materials, hafnium-based materials are particularly desirable due to their relatively low cost and ease of deposition. Prior experimentation with various high-k gate insulation layer materials has revealed that transistors fabricated with HfO2 desirably exhibit a relatively high drive current due to the relatively high k value of HfO2. On the other hand, HfO2 has been shown to be unstable under negative and positive voltage bias conditions, in a phenomenon known as negative or positive bias temperature instability (NBTI/PBTI). HfSiOx, in contrast, has a better thermal stability and exhibits negligible NBTI or PBTI. However, transistors fabricated with HfSiOx undesirably exhibit a relatively low drive current due to the relatively low k value of HfSiOx.
Accordingly, it is desirable to provide improved integrated circuits and methods for fabricating integrated circuits that include hafnium-based gate insulation layer materials. Additionally, it is desirable to provide such integrated circuits and methods for fabricating integrated circuits that exhibit both good temperature stability and a high drive current. Furthermore, other desirable features and characteristics of the present disclosure will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.